The electrolysis of alkali metal halide brines, such as sodium chloride and potassium chloride brines, in electrolytic diaphragm cells is a well known commercial process. The electrolysis of such brines produces halogen, hydrogen and aqueous alkali metal hydroxide solutions. In the case of sodium chloride brines, the halogen produced is chlorine and the alkali metal hydroxide is sodium hydroxide. The electrolytic cell typically comprises an anolyte compartment with an anode therein, a catholyte compartment with a cathode therein, and a liquid permeable diaphragm which divides the electrolytic cell into the anolyte and catholyte compartments. In the foregoing electrolytic process, a solution of the alkali metal halide salt, e.g., sodium chloride brine, is fed to the anolyte compartment of the cell, percolates through the liquid permeable diaphragm into the catholyte compartment and then exits from the cell. With the application of direct current to the cell, halogen, e.g., chlorine, is evolved at the anode, hydrogen is evolved at the cathode and alkali metal hydroxide (from the combination of sodium ions with hydroxyl ions) is formed in the catholyte compartment.
The diaphragm, which separates the anolyte compartment from the catholyte compartment, must be sufficiently porous to permit the hydrodynamic flow of brine through it, but must also inhibit back migration of hydroxyl ions from the catholyte compartment into the anolyte compartment. In addition, the diaphragm should inhibit the mixing of evolved hydrogen and chlorine gases, which could pose an explosive hazard, and possess low electrical resistance, i.e., have a low IR drop. Historically, asbestos has been the most common diaphragm material used in these so-called chlor-alkali electrolytic cells. Subsequently, asbestos in combination with various polymeric resins, particularly fluorocarbon resins (the so-called polymer-modified asbestos diaphragms),have been used as diaphragm materials. Polymer-modified asbestos diaphragms, their preparation and use, are described in U.S. Pat. Nos. 4,065,534, 4,070,257, 4,142,951 and 4,410,411, the disclosures of which are incorporated herein by reference.
More recently, due primarily to possible health hazards posed by air-borne asbestos fibers in other applications, attempts have been made to produce asbestos-free diaphragms for use in chlor-alkali electrolytic cells. Such diaphragms, which are often referred to as synthetic diaphragms, are typically made of non-asbestos fibrous polymeric materials that are resistant to the corrosive environment of the operating chlor-alkali cell. Such materials are typically prepared from perfluorinated polymeric materials, e.g., polytetrafluoroethylene (PTFE). Such diaphragms may also contain various other modifiers and additives, such as inorganic fillers, pore formers, wetting agents, ion-exchange resins and the like. Examples of U.S. patents describing synthetic diaphragms include U.S. Pat. Nos. 4,036,729, 4,126,536, 4,170,537, 4,170,538, 4,170,539, 4,210,515, 4,606,805, 4,680,101, 4,853,101 and 4,720,334. The coating of synthetic diaphragms with various inorganic materials is described in U.S. Pat. Nos. 5,188,712 and 5,192,401.
Chlor-alkali cell diaphragms made principally of asbestos or polymer-modified asbestos generally do not suffer from excessive permeability during start-up of such a cell. However, synthetic diaphragms, as prepared, are generally significantly more permeable at start-up than comparable asbestos diaphragms. This condition leads to low liquid levels in the anolyte compartment using normal brine feed rates. Such "low level" cells, as they are sometimes called, require excessive brine feed and extra operator attention and monitoring.